1. Field of Invention
The invention is directed to a process for producing compression ready a powder mixture of steel powder and to the use of such a powder mixture for fabricating sintered articles with high toughness and density.
2. Description of the Prior Art
The fabrication of mechanical structural component pans from ferrous materials by way of sintering techniques, as opposed to production by cutting or chip-removing machining (e.g., turning, boring, milling), has the great advantage that the actual shaping can be effected in a single work step practically without waste and is therefore faster and more economical for duplicated or series-produced articles. For example, the articles are pressed to form green compacts on a hydraulic metal powder press in a die using a pressing pressure of 7 t/cm.sup.2, for instance, and are then sintered in a furnace at approximately 1120.degree.-1150.degree. C. (normal sintering) or at approximately 1250.degree.-1280.degree. C. (high-temperature sintering) in order to gain a sufficient static and dynamic strength. Owing to conditions of fabrication, the density of the sintered articles is always lower than that of the corresponding solid work material (theoretical density), since the articles are penetrated by pores. In ferrous materials, the actual density of the sintered articles is normally in the range of roughly 80-92% of the theoretical density depending on the applied pressing pressure and the shape of the article. This inevitably leads to impairment of the mechanical properties, of the article. Due to this sintered articles were previously not used under particularly high mechanical stresses, especially since greater dimensioning to compensate for this disadvantage is generally not acceptable due to the resulting increase in volume and weight. In addition, the pores contained in the sintered article can act as inner notches which in particular can lead to a drastic reduction of the dynamic strength characteristics.
In order to reduce the pore volume of sintered articles, it is known to use ferrous base powder with a higher phosphorous content. This leads to noticeable shrinkage during the sintering process and accordingly to an increase in density. The shrinkage of the sintered article is taken into account in the geometrical form of the press die by means of suitable overdimensioning and can accordingly be compensated to a great extent. However, the addition of phosphorous, which can be effected either by appropriate alloying of the melt used in the powder atomization or by admixture of phosphorous compounds with the ferrous base powder, has the disadvantage that it can only be used to a limited extent to increase density, since higher phosphorous contents tend to produce brittleness in the sintered articles and accordingly further increase susceptibility to notching.
Another method for achieving a higher density, i.e., for reducing the pore volume, is the so-called double sintering technique in which the compacted body, after first being sintered generally at a temperature of approximately 700.degree.-900.degree. C., is subjected to another pressing process and a final finish sintering. This is a very cost-intensive process due to the double pressing and sintering.
A ferrous base powder which ensures a comparatively high impact strength is known from WO 91/19582. The prescribed alloying elements compulsorily contain 0.3-0.7 percent by weight phosphorous and 0.3-3.5 percent by weight molybdenum. The sum total of any other alloying elements which may be present is limited to a maximum of 2 percent by weight. The molybdenum content is preferably 0.5 to 2.5 percent by weight and the phosphorous content is preferably 0.4 to 0.6 percent by weight (added in the form of Fe.sub.3 P in particular). A maximum carbon content of 0.07 percent by weight is recommended. This ferrous base powder is suitable for normal sintering temperatures (below 1450.degree. C.). The test results presented in this reference show that there are optimum quantitative proportions for both phosphorous and molybdenum at which the impact strength is especially high. Thus the impact strength increases sharply in a powder with a phosphorous content of 0.5 percent by weight and a molybdenum content of 0 to 1.0 percent by weight, reaches a maximum in the range of 1 to 2 percent by weight, and even drops below the starting value beyond a molybdenum content of 3.5 percent by weight.
Further, DE 29 43 601 C2 discloses a pre-alloyed steel powder for the fabrication of high-strength sintered articles which contains 0.35 to 1.50 % Mn, 0.2 to 5.0% Cr, 0.1 to 7.0% Mo, 0.01 to 1.0 V, a maximum 0.10% Si, a maximum 0.01% Al, a maximum 0.05% C, a maximum 0.004% N, a maximum 0.25% oxygen, remainder iron and other fabrication-related impurities. The low carbon content is required to enable a good compressibility of the steel powder which is produced by water atomization of a corresponding melt and subsequent reduction annealing at 1000.degree. C. Before being compressed to form green compacts, this steel powder is mixed, as is conventional, with lubricants (e.g., 1% zinc stearate) and, in addition, with graphite powder in order to adjust the desired carbon content in the sintered article. The added amount of graphite powder is generally several tenths of a percent (e.g., 0.8%), since the sintered articles are oil-hardened after sintering so as to acquire sufficient strength values. The compression ready metal powder mixture must therefore have a sufficiently high carbon content for a heat-treatable steel while allowing for the anticipated burnup losses during sintering. Due to the carbon content, the sintering process inevitably produces a structure comprising martensite or martensite and bainite or bainite and pearlite, depending on the cooling rate. In order to achieve a density close to the theoretical density of steel, the sintered articles are subjected to a forging process prior to heat treatment.
Toothed gear wheels which are subjected to high mechanical stresses must have a high flank bearing capacity in addition to the highest possible root fatigue strength. Therefore such toothed gear wheels are normally hardened. However, in the case of a work material with relatively high phosphorous content this leads to an unacceptable embrittlement of the structural component part.